Young children are ignorant. They don't know very much. Does that mean they'll never have any useful ideas?
No. They can contribute a lot to a discussion, even though they don't know very much.
The main reason is that although there is a lot they don't know, there are a few areas where they know quite a lot. In particular, they have a lot of knowledge about what they want, and which sorts of situations they would be happy or unhappy with. If you are trying to find a way of proceeding that everyone will consider acceptable and voluntarily agree to, then this knowledge the child has will come in very handy. It will play a major role in figuring out what to do. You couldn't find a way to proceed that everyone likes without some knowledge about the likes of the child.
Children, like everyone else, do not have perfect knowledge. They can be mistaken about what they want, or in their estimations of what future possibilities they would like. And this kind of knowledge is not exclusive to the child. The parent can have some too. But taking those facts into account, it still remains that children have useful knowledge that can help find solutions if it's allowed to.
Let me give a few examples. Suppose the child left out a board game, and it's in the way now. It could be put away in the box, or it could be carried elsewhere to preserve the positions of the pieces. How are you going to know which would be best? You should probably ask the child. And bear in mind he might say something else, like that it's very important to him not to disturb the game, so could it please be left where it is and some other solution found? If he says that, he is contributing important knowledge that's highly relevant to what the best thing to do is. It really is the case that some proportion of the time its important that a game be left undisturbed, and it's good to find out when that is the case or not.
Suppose the child wants a red baloon, but there aren't any more. Which baloon would he like as a replacement? The child probably has the best knowledge of that. And if it's a surprising answer, like he'll accept green baloons but he needs two, or actually if there's no red baloons he'd prefer a water baloon instead, then you'll never get stuff like that right without the child contributing his ideas. And should you go to the store to get more red baloons? That is a question you won't be able to answer accurately without the child contributing some knowledge about how valuable the red baloon is to him (and also the parent contributing knowledge about how inconvenient a store visit would be).
Suppose the child doesn't want to wear his seatbelt. The parent thinks of everything he can to make it better. He gets the child an ipod so he'll have a distraction from the seatbelt. He glues pillows to the seatbelt to make it softer. He paints the seatbelt the child's favorite color. He glues glitter on it. He tells the child stories about seatbelts saving lives, and with heros who like seatbelts. Yet still when he drives he sees his child pushing at the seatbelt, and shifting in his seat, and with a sad look on his face. Finally the parent says: "I give up. Why does the seatbelt bother you so much?" And the child says: "I can't reach the controls to raise the window." And the parent says: "That's so easy to fix. I'll give you a stick you can reach them with from further away. Why didn't you tell me?" And the child says: "It didn't occur to me to tell you, because you didn't ask, and you don't act like my ideas matter." (Or more likely, the child would say "I don't know", but that would be the reason.)